In a conventional configuration of a plasma control system, the output from a number of power generators is combined at a combiner in order to provide a composite RF output signal which is applied to a plasma chamber. Such a configuration enables sufficient power levels to be applied to the plasma chamber, where any one individual power generator could not provide sufficiently high power levels.
In such systems, a main controller must synchronize both the frequency and phase of each power generator. Each power generator must be configured within the system so that the individual power signals output from each power generator possess a desired frequency and phase difference to provide the desired output power in the combined RF output signal. Such synchronization requires an extended start up period. In particular, a first power generator unit is defined as a master power generating unit. The unit outputs a power generation signal having a specific frequency and phase. This master power generator unit defines a starting point to which additional power generation units are slaved in terms of both frequency and phase.
The frequency and phase of each slave power generation unit are then varied in order to provide a combined RF output signal having a desired, typically a maximum, power at a predetermined, desired frequency.
In order to obtain synchronization, a digital logic loop (DLL) is utilized to analyze the combined RF output signal and generate the respective frequency and phases of each slave generation unit in order to achieve the desired power at a predetermined frequency of the combined RF output signal. Tuning the RF power signal using DLL proves to be undesirably timely and extends the start up period of the plasma control system each time the system is restarted because each RF power generation unit requires calibration upon every startup. More particularly, each RF power generation unit must be tuned to account for cable length, connections, and the like in order to output the optimal combined RF power output signal. Further, conventional, multiple DDS systems do not provide for frequency or phase shifting of individual RF power generation units or the combined RF power output signal without recalibrating the master and slave individual RF power generation units using the DLL. This makes such frequency and phase shifting impractical. Accordingly, the output of each individual power generator cannot be adjusted without readjusting the other individual RF power generation units. It is, rather, the combined RF output signal which must be examined in order to determine whether the respective power generator units have been calibrated correctly.